ATHYRIS OF THE UPPER HELDERBERG AND HAMILTON GROUP. 989 



Atliyris vittata. 



PLATE XLVI : Figs. 1-4. 



Mhyris rittala : IUll, Tliirtecntli Report on the State Cabinet, p. 89. 18G0. 

 Compare Jtlhyris concentrica, A. spiri/eroides. etc. 



Shell ovate-subquadrate, gibbons, with the mesial fold and sinus distinct; 

 front con.spicuously sinuate : hinge-line short : cardinal extremities 

 rounded. 



Ventral valve gibbous above, more convex than the dorsal ; umbo pro- 

 minent ; the beak incurved and truncated in the plane of the longitu- 

 dinal axis by a round foramen, curving very abruptly to the cardinal 

 and cardino-lateral margins ; the centre marked by a well-defined 

 mesial sinus, which is continued nearly or quite to the beak, and 

 becoming much deeper and subangularly margined towards the front. 



DoKSAL valve a little less gibbous than the ventral, sides regularly curv- 

 ing; the middle of the upper part distinctly prominent, and developed 

 below in a strong mesial fold which is abruptly elevated in front. 



Surface marked by regularly imbricating lamellose lines of growth, 

 which, on the better preserved surfaces, are finely crenulate on their 

 edges, and the intermediate spaces striate. 

 Interiorly tlie spires of this form, in their first volution and in the 



acces.sary lamella?, are quite distinct from those of A. spir if er aides. The 



accompanjing figure is an illustration of the first volution of the spires, 



with the accessary lamellae. 



Atbtbis vittata. 



[ Pal-eontoi.ogv IV^.] 



Central poriiou of the spires. 



37 



